This longitudinal analysis of employment in 510 Canadian firms over the period 1980 to 1985 provides evidence that union firms in both the manufacturing and nonmanufacturing sectors experienced substantially slower employment growth than comparable nonunion firms. Controlling for industry sector, firm size, and firm age, the author finds that within the manufacturing sector, union firms grew 3.7% more slowly per year than nonunion firms, and within the nonmanufacturing sector, union firms grew 3.9% more slowly than nonunion firms. Small firms in both sectors, however, appear to have escaped any negative union effect on employment growth. Of the control variables, firm age appears to be much more important than firm size in explaining employment growth.The past decade or so has witnessed a dramatic surge of interest in and research on the impact of unions on various aspects of company performance, such as productivity and profitability. Surprisingly, however, relatively few empirical studies have examined the impact of unionization on employment growth within established firms, despite the obvious importance of this issue,